Cavity resonator electric wave generator



y 1948 G. J. LEHMANN 2, 45,784

CAVITY RESONATOR ELECTRIC WAVE GENERATOR Filed June 10, 1943 2 Sheets-Sheet 1 INVENTOR 65mm!) Law/1mm ATTORNEY 'July 27, 1948 G. J. LEHMANN 2,445,784

CAVITY RESONATOR ELECTRIC WAVE GENERATOR v Filed June 10, 1943 2 Sheets-Sheet 2 IINVENTOR Game) (0901mm ATTORN EY Patented July 27,1948

umreo: STATES PATENT OFFICE CAVITY RESONATOR ELECTRIC WAVE GENERATOR Gerard J. Lehmann, New York, N. Y., assignor to International Standard Electric Corporation,

New York, N. Y., a corporation of Delaware 7 Application June 10, 1943, Serial No. 490,319% In France March 11, 1942 1 i a The present invention relates to systcmsof secondary. oscillators for radio electric transmitters of the spark-type with indirect excitation. The invention is characterized, onthe one hand, by an extremely high Q circuit, permitting the pro: longation of the duration of each train of oscillations appreciably beyond the limits heretofore attained, particularly as concerns transmitters functioning at very short Wave lengths, and on the other hand by the absence of constrained dielectricswhich have the disadvantage of preventing the employment of very high exciting voltages.

Even Where the intermittent character of the emission does not present any inconveniences in itself, it is known that the major defect in spark transmitters is the very short duration of each train of oscillations emitted; the duration diminishing with the wave length of the oscillation.

Indeed, in one type of construction with a given circuit, this durationisinversely proportional to the linear resistance of the circuit, which, taking into account the radiation. resistance, increases faster than the square root of the frequency. On the other hand, in circuits with lumped constants, it is hardly possible to utilize at very short wave lengths a type of construction and employin the most desirable circuit with relatively great kinetic energy. Finally for this kind of circuit the duration of the oscillation train diminishes excessively fast with the wave length so that all trafiic then becomes impracticable.

An appreciable improvement of this condition is provided by employing circuits with distributed constants such as transmission lines with parallel conductors Whose axes do not coincide, or better still with concentric conductors. One can thus obtain a relativelyhigh ratio between the kinetic energy SilOI'Bd'il'l the inductance and the energy dissipated in the resistance, a ratio which furnishes the above-mentioned high circuit Q and the desired duration of the train, of oscillations. However, in spite of this undeniable advantage, the value of. the circuit Q is still not sufficient to give, in the majority of applications, an acceptable train-duration, especially at very short wave lengths. Furthermore, the relatively high dielectric strain which is produced between conductors renders diflicult the use of. high voltages at the primary and impedes the utilization of one of the inherent advantages of spark transmitters. Accordingto thecharacteristics of the present invention, there is utilizedfor the secondary circuit. of the-spark emitter, a circuit of the so-called chamber type, or otherwise known as resonant 12 Claims. (Cl. 315-39) cavity. This type of circuitrecently discovered asla result of other uses, presents inthe instant case particular advantages which have already been indicated at the beginning hereof but which will however be recalled because of their importance. i

(a) It lengthens the duration of each train of oscillations because of the exceptional high peak voltage factor, especially at very short Wave lengths. l

(b) It permits in spite of this peak voltage factor, the utilization of veryhigh tensions on the primary because of the absence of strained dielectrics in the interior ,of the secondary. It offers thus a means of forming in association, for example, with the type of primary circuit described in the application of Stanislas Teszner, Serial No. 477,654, filed onMar-ch l, 1943,. under the title System of spark gaps for radio electric emitters, radio electric spark emitters of relatively high energy and efficiency. Application Serial No. 477,654 has become abandoned.

The description which follows, of some examples of embodiments, will produce a better understanding of the characteristics and advantages of the invention. Reference will therefore be successively made to the adjoining drawings in which, i

Fig. 1 serves to illustrate the schematic equivalent of the oscillating circuits of a spark emitter of the indirect excitation type.

Figs. 2 to 5 represent in cross-section, different forms of embodiment of the secondary circuit according to the invention, coupledwith the corresponding primaries bycoaxial lines.

In Fig. 1, it will :be observed that thecircuit constants arerepresented as lumped. The schematic of the primary circuit comprises a capacity CI, an inductance LI and a spark gap E. Represented in the same fashion, thesecondary cormprises an inductance L2and a capacity 02. The indicated coupling, for purposes of illustration, between the primary andsecondary, is electromagnetic, but it can equally Well be of any other kind, such as electrostatic or even mixed.

In the ensuing description, the primary willlbe represented in the form of a line withdistributed constants, of which a coaxial line will be taken particularly as an example. i i

In Fig, 2, such a line comprises an exterior conductor I, a central .conductor 2, closed at one extremity by a spark gap 3 inserted between the two conductors. The line oscillating in fact asa quarter wave line, is coup-led with'a secondary circuit 4 constituted'by a resonant cavity in the form of a cylinder of revolution around the axis AA. The coupling between the two circuits therefore is effected by an antenna forming a spindle diametrically through the cavity 4 and traversing the openings 6 and formed by a prolongation of the central conductor 2 (the said coupling being by half wave). A cap 1 forms a reflector for the energy at the extremity of the antenna. Finally, the insulating tie member or washer 8 insulates the external conductor l and the cap I from the wall of cavity 4. It should be noted that the latter will be constituted preferably of as good an electrical conductor as possible, for example copper, and thatit-can be made in other shapes such as those hereinafter described.

In Fi 3, the resonant cavity constituting the secondary circuit is in the form of a surrounding toroid 9 of rectangular cross-section and surrounding the axis 13-13 of the primary circuit. The latter is represented, as hereinabove, by a coaxial line which however, in this case, oscillates at half wave length. It is formed of an external tube which constitutes at the same time the interior wall of the toroidal enclosure 9, and of a central conductor composed of two truncated sections II and I2 of equal length separated by spark gap l3. The coupling between the primary and secondary is effected through the openings l4 provided in the Wall Ill opposite the spark gap, that is to-say, at its middle portion. The coupling is then almost purely electromagnetic. The amount of coupling is a function of the dimensions and of the number of openings It.

In Fig. 4, the primary circuit is constituted in the same Way as in Fig. 3 (the reference numerals remaining the same for like parts). The cavity constituting the secondary circuit [5 is however in a more complex form. It is given a surrounding toroidal form, the toroid being one of revo' lution around the axis BB of the primary, but the exterior cylindrical wall of which is cut away at its middle portion, the two sections [5 and I1 are prolongations respectively of the annular discs l8 and I9 which are parallel to each other and form what one may call the capacitive par-t of the cavity. The coupling between the primary and the secondary is efiected as in Fig. 3, through the openings M which are situated preferably in the space between the two discs l8 and [8. The coupling is almost purely electromagnetic and the amount of coupling is likewise determined by the dimensions of the openings It.

In Fig. 5, finally, the secondary cavity 26 is formed of a surrounding toroid (toroid of revolution around the axis B-B of the primary) but Whose interior wall is separated at its middle part into two sections 2|, 22. The free extremities of these sections are extended respectively by two annular parallel discs 23, 24, which form, likewise, the capacitive part of the circuit. The primary circuit is constituted as in Figs. 3 and 4 by a coaxial line; however the exterior conductor of this line forms also the interior wall of the toroidal cavity, and is interrupted at its middle portion as indicated. in the figure. The coupling between the primary and the secondary is efiected in the space between 23, 24, common in part to the two circuits. The coupling is principally, but not exclusively, electromagnetic and its value is a function of the length of the said space between 2.3, 24.

The functioning of the embodiments represented will be easily understood. The primary circuit being charged by a source of energization, not indicated in the drawing (the voltage being applied in Fig. 2 between the conductors I and 2, in Figs. 3, 4 and 5, between the sections I l and l 2 of the central conductor), when the spark gap is fired, there is provoked an oscillation of the primary circuit which excites in its turn that of the secondary cavity. The oscillation of the primary being then rapidly extinguished by an appropriate arrangement, such for example as that disclosed in the said application of Stanislas Teszner referred to herein'above; the secondary circuit will thereafter continue to oscillate by itself. It is then that the inherent quality of the resonant cavities intervenes, namely their exceptionally high Q circuit. This is of a particular interest because it insures a relatively long duration of the secondary oscillation. In order to fix the orders of dimensions, it can be said that with resonant cavities one can easily attain a circuit Q value of the order of 10,000, and even more for wavelengths of the order of a meter. Assuming then a utilization coeflicient in the emitting antenna (which is excited from the secondary for example, in a manner wellknown by means of a loop introduced into the interior of the cavity, which loop is not represented in the drawings) of the order of 0.9, it is easy to see that the duration of the oscillation until there is a reduction of amplitude of 40 decibels, will correspond to about 1,500 alternations. It is evident besides that one will be able without any special disadvantage at the secondary, to utilize at the primary, charging voltages which are very high and with relatively valuable useful energies in the antenna.

With respect to the choice of the form of the secondary cavity, it will be a function of the particular conditions applying to each particular species, notably, the high circuit Q, the wave length of the oscillation, the loading of the circuit, ease of operation, conditions of installation, etc, as well as the constitution of the primary circuit.

It will be understood, of course, that the abovedescribed forms have been given only by way of example and not by way of limitation, and that they can be varied without departing from the spirit and scope of the invention.

What is claimed is:

1. An oscillation generator for very short wave lengths comprising a conductive resonant cylindrical cavity, an antenna-like member passin through said cavity substantially diametrically therein but conductively insulated therefrom, an integral prolongation of said member extending outwardly beyond said cavity, a concentric transmission line conductor surrounding said prolongation and forming therewith a quarter-Wave concentric transmission line, and a spark gap interposed between the two conductors of said line exterior to said cavity space.

2. An oscillation generator according to claim 1 in which both ends of the antenna-like mem-' ber extend beyond the cavity member and each end is "enclosed by a metal cap spaced therefrom to form a concentric line, and a spark gap mem pled to said primary circuit but galvanically.

insulated from said conductor, the extent ofithe resonator in the direction of said conductor being of the order of half a wave length at the operating frequency.

4. An oscillation generator according to claim 3, wherein the resonator is of toroidal form and coaxial with said linear conductor.

5. An oscillation generator according to claim 3, wherein the resonator is in the form of a hollow cylinder having its axis at right angles to said linear conductor.

6. An oscillation generator according to claim 5, wherein a portion of said linear conductor passing diametrically through said hollow cylinder is extended externally thereof for a distance corresponding to one-quarter the wave length at the operating frequency, the said spark gap being provided at the end of said extension.

7. An oscillation generator according to claim 6, further comprising a tubular conductor surrounding said extension and forming a quarterwave concentric transmission line therewith, the spark gap being provided between said extension and said tubular conductor.

8. An oscillation generator for use at very short wave lengths, comprising a primary circuit which includes a linear conductor, said conductor having a discontinuity forming a spark gap, and a secondary circuit formed by a toroidal cavity resonator coaxial with said conductor, the extent of said resonator in axial direction on each side of said spark gap being of the order of a quarter wave length at the operating frequency.

9. An oscillation generator according to claim 8, wherein the toroidal resonator has an outer wall formed with axial reentrant portions surrounding respective portions of said linear conductor on each side of said spark gap, each of said reentrant portions extending almost to the center of the resonator.

10. An oscillation generator for use at very short Wave lengths, comprising a primary circuit which includes a linear conductor, said conductor having a discontinuity forming a spark gap, and a secondary circuit formed by a toroidal cavity resonator an inner wall of which coaxially surrounds said linear conductor and has an air gap adjacent the said spark gap, the extent of said resonator in axial direction on each side of said spark gap being of the order of a quarter wave length at the operating frequency.

11. An oscillation generator for use at very short wave lengths, comprising a primary circuit which includes a linear conductor, said conductor having a discontinuity forming a spark gap, and a secondary circuit formed by a toroidal cavity resonator an inner wall of which coaxially surrounds said linear conductor and has a plurality of circumferentially spaced apertures substantially co-extensive with the said spark gap, the extent of said resonator in axial direction on each side of said spark gap being of the order of a quarter wave length at the operating frequency.

12. An oscillation generator according to claim 11, wherein the toroidal resonator comprises an outer wall having a gap aligned with said circumferentially spaced apertures, a pair of conductive annular members extending inwardly from said outer wall on each side'of said gap.

GERARD J. LEI-IMANN.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 706,739 Fessenden Aug. 12, 1902 835,023 Heinicke Nov. 6, 1906 1,304,868 Franklin May 27, 1919 1,407,061 Gray Feb. 21, 1922 2,129,713 Southworth Sept. 13, 1938 2,190,668 Llewellyn Feb. 20, 1940 2,240,941 Ohl May 6, 1941 2252293 Ohl Aug. 12, 1941 

